FIG. 5 illustrates a conventional communication circuit 160 using a power coupler 172 to measure the power of an amplified signal 170. This conventional communication circuit 160 measures transmitted power in the presence of load mismatch (VSWR, or Voltage Standing Wave Ratio), but has several major drawbacks including: large area, large cost, and introduction of significant insertion loss and efficiency degradation because a fraction of the amplified signal is diverted to a sensed signal through the power coupler.
Specifically, input node 162 sends input signal 164 to amplification circuit 166. Amplification circuit 166 includes at least one amplifier 168, and also sends amplified signal 170 to power coupler 172.
Power coupler 172 effectively splits amplified signal 170 into output signal 174 and sensed signal 178. Output signal 174 is sent towards output node 176. Sensed signal 178 draws a substantial amount of power from amplified signal 170, effectively attenuating amplified signal 170 to generate output signal 174. In other words, the power of output signal 174 plus sensed signal 178 approximately equals the power of amplified signal 170.
Sensed signal 178 is sent towards correction circuit 180. Correction circuit 180 may perform signal processing, and may send control signal 182 towards amplification circuit 166. Thus, correction circuit 180 forms a feedback loop, although not necessarily a classic feedback loop. Classic feedback is defined as measuring an output of a system, comparing the output to a reference (such as an input signal), generating an error signal based upon the comparison, and then controlling the system based upon the error signal.
Conventional communication circuit 160 suffers from many additional problems. First, a substantial amount of power is drawn by away by sensed signal 178.
Second, sensed signal 178 is sensitive to distortions caused by, for example, parasitic coupling to output node 176 which in many cases is a relatively large trace that travels on the module board alongside of the signals before going to correction circuit 180.
Third, amplified signal 170 is a high power signal, and therefore power coupler 172 must have a very high linearity in order to handle high power. Furthermore, in order to operate properly the power couplers need to have their size a significant fraction of the processed signal wavelength. This results in very large sizes for the power couplers when the processed signals have low frequencies (large wavelengths).
In conventional circuits, sensing an amplified signal is difficult due to large load mismatches that vary slowly in time (VSWR). These load variations result in wide variations in the amplified signal, and these wide variations are costly to correct.
The total output phase from an amplified signal may be separated into two components: a quasi-static component and a dynamic component. The quasi-static (slowly varying) component is a function of the VSWR load, but is constant with respect to power. In other words, the quasi-static component remains constant for a given load, even if the power increases.
In contrast, the dynamic component varies strongly as a function of power. Thus, linearizing the response of a power amplifier circuit generally only requires compensating for the dynamic component, in order to correct for the dynamic phase variation. However, if the quasi-static and dynamic components are not separated, then a very wide range is required from the correction circuit, which results in the correction circuit having large area, high cost, and high power dissipation.